The present invention relates to an inner box for a cooker formed by bonding a plurality of sheets of plate members, particularly relates to an improvement in a bonding portion of a face plate.
Conventionally, a plate constitution of an inner box for a cooker is constituted by a front plate 51 having an opening portion, a rear plate 53, an inner box main body 55 provided between the front plate 51 and the rear plate 53, and a ceiling plate 57 covering from an upper face thereof as generally shown by FIG. 9.
In such an inner box main body 55, as shown by a bonding portion for bonding the front plate 51 and a face plate 61 of the inner box main body 55 in FIG.10(a), an end edge of the opening portion of the front plate 51 is formed with a fold-to-bend portion 59, and a fold-to-bend portion 63 constituting a V-like shape relative to a face of the front plate 51 and skewedly projected to the face plate 61 of the inner box main body 55 is formed from the fold-to-bend portion 59. Meanwhile, also at an end portion of the face plate 61 of the inner box main body 55, similar to the front plate 51, an end edge thereof is formed with a fold-to-bend portion 65 and a front end portion 65a thereof is arranged on an inner side of the fold-to-bend portion 63 to prepare for a calking step.
Further, as shown by FIG. 10(b), in the calking step, in a state of supporting a lower side in the drawing of the front plate 51, while inserting the front end portion 65a of the fold-to-bend portion of the face plate 61 into a space between the fold-to-bend portion 63 of the front plate 51 and the front plate 51, a calking jig, not illustrated, is pressed from above the face plate 61 to the front plate 51 over an entire periphery of the bonding portion for bonding the face plate 61 and the front plate 51. Thereby, the fold-to-bend portion 63 of the front plate 51 and the fold-to-bend portion 65 of the face plate 61 are brought into close contact with each other by being sandwiched by each other to thereby fix the front plate 51 and the face plate 61 of the inner box main body by end calking (refer to, for example, Japanese Utility Model Publication JP-UM-A-59-172904).
However, according to the above-described inner box for a cooker, four sheets of the face plates 61 to be bonded with the front plate 51 of the inner box main body 55 are simultaneously calked to bond to the front plate 51 and therefore, a pressure applied to the calking jig is considerable, further, also an equipment for the calking step becomes large-sized. Therefore, when the face plates 61 are successively bonded to the front plate 51 sheet by sheet instead of simultaneously calking to bond four sheets of the face plates 61 thereto, the calking step can be carried out simply and conveniently without applying the considerable pressure on the calking jig, however, a bonding structure having the above-described constitution poses the following problem.
That is, in pressing the face plate 61 of the inner box main body 55 by the calking jig, there is a play between the respective fold-to-bend portions 63, 65 in a pressing direction of the front plate 51 in calking, that is, both of the fold-to-bend portions 63, 65 are projected in height directions and therefore, a slide amount ΔH in the pressing direction of the face plate 61 is prolonged by pressing in calking. Therefore, when the calking step is carried out in a state of respectively dividing four sheets of the face plates 61 to be bonded with the front plate 51 of the inner box main body 55, the above-described difference in height of ΔH is produced between the face plate after the calking step and the face plate before the calking step contiguous thereto. By the difference in height, warping is produced in the inner box main body 55 bonded with four sheets of the face plates and also the front plate 51 is bent. When the calking step is continued in a state in which the respective face plates and the front plate are respectively bent in this way, the inner box 55 is finished in a state of the inner box 55 being warped, bent are buckled.
Further, there is a case of carrying out a surface treatment of, for example, precoated metal (PCM) or the like on a surface of the front plate 51 and surfaces of the respective face plates 61 of the inner box 55 to meet a request of promoting a beautiful outlook in household electric appliances in recent times. In this case, since the precoated metal is provided with insulating performance, when the faces which are brought into close contact with each other by the calking step are constituted only by insulating faces, the respective face plates 61 are brought into an unconducted state and when a high frequency wave is irradiated into the inner box 55 for cooking, radio wave shielding of the inner box 55 becomes incomplete and leakage of radio wave is brought about. Therefore, shapes of the fold-to-bend portions 63, 65 are restricted in view of design since the respective face plates 61 need to be connected electrically.